This is a new mesh recolour, and means that you will need to download the particular mesh itself in order for this to show up in your game. See the Meshes Needed section or read the text for more information.

Shiny cocktail dresses in 7 colors(avocado, chocolate, magenta, midnight, red, purple and aqua) They come both in everyday and formal category.
Mesh is by Marvine, you can find it here: http://www.modthesims2.com/showthread.php?t=127591, thanx to Marvine for creating this wonderful mesh!
Enjoy!